The present invention relates to an apparatus for regulating the tension of a strap when it has been wound around a package in a package strapping machine.
In conventional package strapping machines, a thermoplastic strap is looped around a package, and the leading end of the strap is clamped. Then the strap looped around the package is tightened, and an intermediate portion of the strap is cut, and then the overlapping ends of the strap are heated, pressed together and welded so as to cause the package to be strapped.
In such conventional machines, the strap is fed to a strap guiding frame so as to be looped around the package, and the leading end of the strap is clamped. Then the strap is preliminarily tensioned so that it may be detached from the strap guiding frame and come to be directly wound around the package. Then the strap is fully tensioned around the package, and the overlapped portions of the strap are clamped. Then an intermediate portion of the strap is cut, and the overlapping ends of the strap are pressed together and welded. A cycle of the sequential operation ends with the next feeding the strap to the strap guiding frame.
In order to meet the needs of the full tensioning process, it is most common to dispose a strap tensioning mechanism along the path to be traveled by the strap, i.e., midway between a reel from which the strap is delivered and a table on which the package is placed. The strap tensioning mechanism includes a pair of rollers by which the strap is pulled back toward the reel when the strap has been directly wound around the package. One of this pair of rollers, by means of its connection with a spring, is urged toward the other so as to hold the strap therebetween during the full tensioning process. The strap tensioning mechanism further includes a strap conduit pivotally mounted on a pin so as to be pivoted in the vertical direction when the tension of the strap passing therethrough is changed. The upward momentum of the strap conduit is utilized for detecting the tension of the fully tensioned strap. The strap conduit is urged downwardly by a spring.
The trouble is that the tension of the strap to be obtained in the full tensioning process varies with the kinds of packages and straps. In order to change the tension of the strap so as to adjust it to a specific kind of package or strap, it has been a common practice that the tension of the spring by which one of the pair of rollers is urged toward the other and the tension of the spring by which the strap conduit is urged downwardly are changed separately. However, the work of separately changing the tension of two springs gives rise to difficulties and is time-consuming, and yet the adjustment can be made only with low accuracy.
For example, a switch for stopping the motor which is driving the rollers will be actuated before the tension of the strap comes up to a predetermined degree if the tension of the spring by which one of the pair of rollers is urged toward the other is increased to a disproportionately higher degree that the increase in the tension of the spring by which the strap conduit is urged downwardly. On the other hand, the rollers will slip on the surfaces of the strap during the full tensioning process if the tension of the spring by which the strap conduit is urged downwardly is increased to a disproportionately higher degree than the increase in the tension of the spring by which one of the pair of rollers is urged toward the other.